1. Field of Art
The disclosure generally relates to the field of electronic paper displays. More particularly, the invention relates to systems and methods for dynamically creating waveform palettes that include representation of waveforms for driving pixels in a display region.
2. Description of the Related Art
Several technologies have been introduced recently that provide some of the properties of paper in a display that can be electronically driven to display various objects. Some of the desirable properties of paper that this type of display tries to achieve include: low power consumption, flexibility, wide viewing angle, low cost, light weight, high resolution, high contrast and readability indoors and outdoors. Because these types of displays attempt to mimic the characteristics of paper, they are referred to as electronic paper displays (EPDs) in this application. Other names for this type of display include: paper-like displays, zero power displays, e-paper and bi-stable displays.
A comparison of EPDs to Cathode Ray Tube (CRT) displays or Liquid Crystal Displays (LCDs) reveal that in general, EPDs require less power and have higher spatial resolution; but have the disadvantages of slower update rates, less accurate color control and lower color resolution. Many electronic paper displays were previously only grayscale devices. Color EPDs are becoming available although often through the addition of a color filter, which tends to reduce the spatial resolution and the contrast.
The key feature that distinguishes EPDs from LCDs or CRTs is that EPDs can maintain an image without using power. They are sometimes referred to as “bi-stable” because black or white pixels can be displayed continuously and power is only needed to change from one state to another. However, some devices are stable at multiple states and thus support multiple colors without power consumption. EPDs are also typically reflective rather than transmissive. Thus they are able to use ambient light rather than requiring a lighting source in the device. Various technologies have been developed to produce EPDs. Depending on the technology used, such displays are sometimes called electrophoretic displays, electro-wetting displays, or cholesteric LCD (Ch-LC). Techniques have also been developed to produce EPDs by embedding organic transistors into flexible substrates.
The luminance or color of a pixel in a traditional LCD display depends on the voltage currently being applied at the given point, with a given voltage reliably corresponding to a specific luminance. The luminance or color of a pixel in a bistable display, on the other hand, typically changes as voltage is applied. For example, in some bi-stable displays applying a negative voltage to a pixel makes it lighter (higher luminance) and a positive voltage makes it darker. The higher the voltage and the longer or more times that voltage is applied, the larger the change in luminance. This has two implications for driving such displays. First, electronic paper displays are typically controlled by applying a sequence of voltages to a pixel instead of just a single value like a typical LCD. These sequences of voltages are sometimes called waveforms. The second implication is that the control signals used to drive a pixel depend not only on the optical state to which the pixel is being driven, but also on the optical state from which it is being driven. Depending on the display technology, other factors may also need to be taken into consideration when choosing the waveform to drive a pixel to a desired color. Such factors can include the temperature of the display, optical state of the pixel prior to the current optical state and dwell time (i.e. the time since the pixel was last driven). Failure to take these factors into account can lead to faint remnants of images that have supposedly been erased still being visible, a visual artifact known as ghosting. Some displays also have additional requirements that must be met to avoid damaging the display, such as the requirement that waveforms be DC balanced.
To handle these issues, some controllers for driving the EPDs are configured like an indexed color-mapped display. The frame buffer of these EPDs includes an index to the waveform used to drive that pixel instead of the waveform itself. Whenever the optical state of a pixel is to be changed, the index of the appropriate waveform is chosen based on at least some of the factors listed above, and the pixel's location or the frame buffer pixel in the frame buffer is set to that index. Some displays encode some factors (such as a pixel's current and desired optical state) in the waveform index and then choose which waveform table to drive a set of pixels based on other factors (such as temperature).
A drawback of the approach used by these controllers is that while waveforms for a first update are being applied to the display, these controllers cannot apply a second update specified by an end user application. These controllers must wait until the first update is completed before starting a second update. This introduces unacceptable latencies in changing pixel values for rendering.